A huge market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable head for reading and writing data. The HSA forms part of a servo control system that positions the moveable head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes one or more disks in which each disk has a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the head.
The head typically comprises a read/write transducer formed on the trailing surface of a slider. When the disk media is rotated, a thin film of air forms between the disk and an air bearing surface (ABS) of the slider. During operation of the disk drive, the head is said to “fly” over the surface of the disk, with the ABS being disposed just above the disk's surface. The thin film of air formed between the ABS and the disk surface is known as the air bearing. The very small separation distance between the transducer of the flying head and the surface of the disk is referred to as the “fly height”. When the flying head is suspended above the disk in this manner, it is moved by the servo control system over a desired concentric track of the disk to access data stored on that track.
The fly height of the head is a factor affecting the density of magnetic data that can be stored on the disk. In recent years, the magnetic recording industry has strived to increase data storage density by employing various techniques aimed at decreasing the average fly height of the head over the rotating disk. Dynamic fly height (DFH) heads are utilized to fly at increasingly smaller fly heights to increase data storage capacity.
Typically, to control the fly height of a DFH head relative to a disk, power is applied in the form of current to a heater element of the DFH head which causes the DFH head to move closer to the disk. In this way, the DFH head is able to fly at a predetermined distance from the disk in order to read and write magnetic patterns to the disk. As storage capacity has increased, DFH heads are required to fly closer to disks and to maintain smaller more precise distances from the disks.
In order to characterize a DFH head to determine an optimal fly height, characterization testing is performed to characterize the fly height of the DFH head across an applied power range. These characterization methods typically rely on spacing models that are utilized to estimate head media spacing (HMS) in terms of an HMS curve. Typically, a Wallace spacing model is used. The Wallace spacing model or Wallace spacing loss equation, expresses a relationship between the read-back voltage from the head and head/disk spacing.
In particular, the Wallace spacing loss equation describes the amplitude of the read-back signal to the spacing of the head above the recording medium (HMS), as follows:
      HMS    =                  λ                  2          ⁢          π                    *              ln        ⁡                  (                                    V              1                                      V              0                                )                      ;wherein HMS is the fly height of the head above the disk; λ is the wavelength in distance between two magnetic data transitions; V1 is the amplitude of the read-back signal at distance HMS; and V0 is the amplitude when the head is very close to the disk or if the head is making contact with the disk (i.e., the touchdown voltage).
A typical assumption for the Wallace spacing loss equation for the generation of HMS curves is that the measured head voltage is a linear function of the magnetic field sensed by the DFH head. Unfortunately, if the linearity assumption between the magnetic field and the read-back voltage is incorrect, the spacing measurements may lead to an erroneous determination of head/disk spacing (i.e., the fly height).
The Wallace spacing loss equation is typically utilized in the testing of DFH heads to develop an HMS actuation curve for a group of heads to determine a proper operating fly height for the DFH heads. However, the reliability of this method is premised upon the fact that the DFH heads act in a linear fashion. If a DFH head has non-linear characteristics, then the Wallace spacing loss equation may not accurately describe the DFH head and the HMS actuation curve that allegedly describes the fly height characteristics of the DFH head may likewise be inaccurate.
Unfortunately, if the fly height for non-linear DFH heads is inaccurately determined, head-disk interactions and damage to the heads and/or disks may occur during operation. It is therefore desirable to identify non-linear DFH heads.